Re: [PATCH v2 01/47] kernel: Add support for poweroff handler call chain

[Guenter Roeck]

On 10/20/2014 11:46 PM, Philippe RÃtornaz wrote:
> Hello
>
> [...]
> > - Use raw notifiers protected by spinlocks instead of atomic notifiers
> [...]
>
> > +/**
> > + *    do_kernel_power_off - Execute kernel poweroff handler call chain
> > + *
> > + *    Calls functions registered with register_power_off_handler.
> > + *
> > + *    Expected to be called from machine_power_off as last step of
> > + *    the poweroff sequence.
> > + *
> > + *    Powers off the system immediately if a poweroff handler function
> > + *    has been registered. Otherwise does nothing.
> > + */
> > +void do_kernel_power_off(void)
> > +{
> > +    spin_lock(&power_off_handler_lock);
> > +    raw_notifier_call_chain(&power_off_handler_list, 0, NULL);
> > +    spin_unlock(&power_off_handler_lock);
> > +}
>
> I don't get it. You are still in atomic context inside the poweroff callback
> since you lock it with a spinlock.
>
> It does not change much from the atomic notifier which was doing exactly the
> same thing but with RCU:
>
>     rcu_read_lock();
>     ret = notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls);
>     rcu_read_unlock();
>
> Why not using the blocking_notifier_* family ?
> It will lock with a read-write semaphore under which you can sleep.
>
> For instance, twl4030_power_off will sleep, since it is doing I2C access.
> So you cannot call it in atomic context.

Learning something new all the time. I assumed that spin_lock (unlike
spin_lock_irqsafe) would not run in atomic context.

I did not want to use a sleeping lock because I am not sure if that
works for all architectures; some disable (local) interrupts before
calling the function (eg arm and arm64), and I don't want to change
that semantics.

I have another idea how to get there without changing the lock situation
while executing the call chain - just set a flag indicating that it is
running and execute it without lock. Would that work ?

Guenter

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/